The simplified reference tissue model for SPECT/PET brain receptor studies

Summary

Aim: The SRTM (simplified reference tissue model) of brain receptor imaging assumes that the time activity curve in the receptor-rich region of interest can be fitted satisfactorily by the 1-tissue compartment model. This assumption has been formulated by a rather restrictive constraint on the rate constants. Empirically, the SRTM might well describe also tracers which do not fulfil this constraint, such as [¹¹C]raclopride, for example. However, this has not been justified rigorously. Methods: The requirements for the SRTM to be applicable are analyzed in detail. Results: The SRTM is applicable under a less restrictive constraint than described previously. The interpretation of the SRTM parameters R₁ and K₂ in physiological terms depends on the constraint, while the interpretation of BP_ND does not. Conclusion: Correct interpretation of the results of the SRTM is tracer specific. In particular, the parameter R₁, which in case of compliance with the original constraint might be used to detect perfusion and/or extraction effects, might not be appropriate for this purpose in case of raclopride-like tracers.

Zusammenfassung

Ziel: Die Vereinfachung der Operationsgleichung des vereinfachten Referenzgewebemodells (SRTM) der Neurorezeptorbildgebung beruht auf der Annahme, dass die Zeitaktivitätskurve in den interessierenden rezeptorreichen Gehirnregionen hinreichend gut durch ein 1-Gewebe-Kompartment-Modell beschreibbar ist. Formelmäßig wurde dies durch eine restriktive Nebenbedingung für die Ratenkonstanten in das Modell eingeführt. Die Erfahrung zeigt, dass mit SRTM auch die Kinetik von Tracern beschrieben werden kann, die diese Nebenbedingung nicht erfüllen, z. B. [¹¹C]Racloprid. Der Grund hierfür ist bisher nicht untersucht. Methoden: Wir beschreiben die Bedingungen für die Anwendbarkeit des SRTM. Ergebnisse: Das SRTM ist auch unter einer weniger restriktiven Nebenbedingung für die Ratenkonstanten anwendbar. Die physiologische Bedeutung der SRTM-Parameter R₁ und K₂ hängt jedoch von der Nebenbedingung ab. Dies gilt nicht für BP_ND. Schlussfolgerung: Die korrekte Interpretation der Ergebnisse aus der Anwendung des SRTM ist tracerspezifisch. So kann z. B. der Parameter R₁, der bei Gültigkeit der restriktiven Nebenbedingung zur Detektion von Perfusions-und/oder Extraktionseffekten genutzt kann, bei Racloprid-artigen Tracern nicht ohne Weiteres dazu eingesetzt werden.

Keywords

Receptor imaging - tracer kinetic modeling - simplified reference tissue method - compartment model - positron emission tomography - raclopride

Schlüsselwörter

Neurorezeptor-Bildgebung - kinetische Modellierung - vereinfachtes Referenzgewebemodell - Kompartmentmodell - Positronenemissionstomographie - Racloprid

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